Detectors are replicated on a single insulating substrate (i.e., glass) during fabrication, using a sequence of photomasks and associated processing, as is well known in the art. Detectors on the substrate are separated from one another by the guard ring of each of the detectors. Near the end of the fabrication process, the substrate is scribed and the individual detectors are separated from one another. The detector is comprised of the active area, which is in turn comprised of individual picture elements (or pixels), contact fingers located on the periphery on one or more sides of the active area, and the guard ring. The guard ring forms a perimeter around both the contact fingers and the active area.
The guard ring may be comprised of one or more materials, at least one of which will be conductive. Because the devices which comprise the pixels in the active area are very sensitive to electrostatic discharge (ESD), which may alter their function or destroy them altogether, they must be protected from discharge, particularly during and after separation when the detectors are subject to ESD from handling. The guard ring affords some protection in that its width forms a gap between the edge of the detector which is handled and the contact fingers which form part of a very conductive path directly into the sensitive active area of the detector. Furthermore, the guard ring can be contacted to a known potential (ground, for example) during the fabrication and detector assembly steps which occur after separation (test, for example) and when it is assembled to the associated support electronics in the intended end product. This will further reduce the chance that ESD will damage the devices comprising part of the pixels. Contacting the guard ring to a known potential also indicates the guard ring should be conductive.
The ESD sensitivity of the devices forming part of the pixels in the active area often can not be predicted before they are first fabricated. A device which is very sensitive to ESD will benefit from more protection (i.e., a wider guard ring). The optimum guard ring width cannot be determined during the initial design, meaning that the design, including the sensitive devices and the guard ring width, must be fabricated and tested before the guard ring width can be evaluated. This implies that the final design will be generated only after one or more design iterations have been fabricated and tested. This further implies that the time to complete the design is increased as is the cost of generating additional iterations of the photomasks in order to optimize the guard ring width. A photolithographic exposure tool may be used to position each guard ring. Lastly, the guard ring width cannot be made arbitrarily large because for smaller detectors fabricated on larger substrates, this would imply fewer detectors per substrate are produced, which also would adversely affect cost.
It would be desirable, then, to have a means for providing a variable guard ring width between detectors on a substrate, which guard ring width can be varied for the purpose of experimentally determining the optimal width for ESD protection without the time and expense associated with regenerating the photomasks.